Simplified flat mirror scanner

ABSTRACT

A system is disclosed for translating a collimated beam of light, such as that generated by a laser, into a raster scan on a planar target. The laser beam is directed at a first rotating mirror provided with a plurality of faces from which it is redirected toward a first stationary mirror with the first rotating mirror causing a linear sweep across the concave face of the first stationary mirror. From the first stationary mirror, the laser beam is directed toward a second multiface rotating mirror disposed beyond the focal point of the first stationary mirror. The second rotating mirror imparts a second scan direction to the laser beam which is substantially normal to the first scan direction. From the second rotating mirror, the laser beam is redirected toward a second stationary concave mirror from which it is redirected in a raster scan to a planar target disposed at a distance from the secondary stationary mirror at which the beam is substantially in focus. A line scan embodiment of the invention is achieved by directing the beam from the laser directly onto the second rotating mirror.

1 Jan. 1,1974

U uncu D mulch Buck [5 SIMPLIFIED FLAT MIRROR SCANNER of light, such asthat generated by a laser, into a raster [76] Inventor, Willard E BuckBox 67' scan on a planar target. The laser beam is directed at a firstrotating mirror provided with a plurality of 4 Lake Havasu Clty' Am 8603 faces from which it is redirected toward a first station- Oct. 12,1971 Appl. No.: 188.115

[22] Filed: ary mirror with the first rotating mirror causing a lin lear sweep across the concave face of the first stationary mirror. Fromthe first stationar y mirror. the laser beam is directed toward a secondmultiface rotating mirror disposed beyond the focal point of the first[52] US. 350/7, l78/7.6 350/294 [5]] Int. CI. 602i) 17/00 [ionary mirrorThe second rgtating mirror imparts a second scan direction to the laserbeam which is substantially normal to the first scan direction, From[58] Field of Search the second rotating mirror, the laser beam isredirected toward a second stationary concave mirror from which it isredirected in a raster scan to [56] References Cited UNITED STATESPATENTS a planar target disposed at a distance from the secondar mirrorat which the beam is substantial] l 350 7 me scan embodiment of themventro Bousky...i.......r. .i...... Kennedy.......im...r...,....Corcoran directing the beam from the laser directly onto the secondrotating mirror.

Primary Examiner-Ronald L. Wibert Assistant Examiner-Michael J. TokarAzt0rney-William C. Cahill et al.

5 Claims, 1 Drawing Figure NTEUJAM 1 I974 INVENTOR AZTOENEYS SIMPLIFIEDFLAT MIRROR SCANNER This invention relates to scanning apparatus and,more particularly, to apparatus for developing a raster scan on a planarsurface from a collimated beam of v light.

Optical systems for translating a collimated beam of light into a rasterscan are known in the prior art. However, the prior art systems aregenerally characterized either by considerable complexity or by limitedperformance, More specifically, the high performance prior art systemsutilize a plurality of rotating mirrors which may have many facesdisposed circumferentially about the axis of rotation. In order toachieve a high scan rate, many faces are required to avoid a cumbersomesize which causes problems at high rotational rates. Conversely, it isdifficult and expensive to achieve a multiface mirror within thenecessary tolerances so long as the physical size is restricted. Aproblem which has been associated with the more modest system is thatthe raster scan achieved is typically not focused across the entiresurface of a planar target and,

v in fact, can only be brought into focus on a curved target area.Therefore, it will be appreciated that it would be desirable to providea relatively simple optical system for translating a collimated beam oflight into a planar raster scan.

It is therefore a broad object of this invention to provide an improvedscanning apparatus for translating a collimated beam of light into aplanar raster scan.

It is a further object of this invention to provide such scanningapparatus utilizing optical elements which are relatively simple and maybe economically produced.

It is a still further object of this invention to provide such scanningapparatus in which the rotating mirrors require a modest number of facesand are therefore relatively easy to prepare within the necessarytolerances to achieve minimum dead time and maximum uniformity from faceto face.

Yet another object of this invention is to provide scanning apparatus inwhich all elements of the scanning system are reflective and thereforeperforms equally as well with light of different wave lengths.

The invention in its various manifestations will become more readilyapparent to those skilled in the art through a perusal of the followingspecification taken in conjunction with the subjoined claims and thesingle FIGURE which is a perspective view of a preferred embodiment ofthe invention.

Referring now to the single FIGURE, it will be observed that acollimated beam of light 1 is directed from a source, such as the laser2, to one side of the axis of rotation of a pyramidal mirror 3. As willbe understood from the description that follows, the pyramidal mirror 3,by virtue of its rotation, imparts the vertical scan function to thecollimated beam in its translation into a raster swet ,3. Hence, thoseskilled in the art will appreciate that t e pyramidal mirror 3 need notrotate at extremely high peripheral velocities nor does it require avery high number of faces. In the embodiment presented, the pyramidalmirror 3 is provided with six equally spaced faces, each disposed at anangle of 45 with respect to the axis of rotation. Hence, the collimatedbeam of light 1 is redirected through an angle of 90 and, as thecollimated beam 1 impinges on each successively passing face of thepyramidal mirror 3, a vertical sweep through an angle of 30 is achieved.As-

suming rotation in the direction indicated by the arrow on the motor 4,the sweep will be repetitively downward with very little dead timebecause of the abrupt definition between adjacent faces of the pyramidalmin ror 3.

The sweeping beam of light, represented by the dashed lines, reflectedfrom the faces of the pyramidal mirror 3 is directed to the face of aconcave stationary mirror 5 which has suitable height to accommodate theentire sweep imparted by the rotating pyramidal mirror 3. The stationaryconcave mirror 5 has a face curvature approximating a spherical sectionor a symmetrical section from the minor diameter of an ellipticalspheroid.

The stationary mirror 5 is canted slightly with respect to the planedefined by the beam sweeping between the rotating pyramidal mirror 3 andthe stationary mirror 5 such that the beam reflected from the stationarymirror 5 passes just clear of the pyramidal mirror 3 as shown in thesingle FIGURE.

As previously noted, the stationary mirror 5 is either a sphericalsection or a section from an elliptical spher oid such that the beam isbrought to a focus at a point 6 which is at a distance from the surfaceof the stationary mirror 5 related to the curvature of the surface inaccordance with principals well known in the art. After the beam oflight passes through the point 6, which will be one of a family ofpoints described as the beam sweeps vertically, the beam diverges. Theposition of the point 6 is predetermined to occur at a position whichwill ultimately result in the beam being focused on the target as willbe more readily appreciated from the remainder of the description.

The laser beam, diverging after passing through the point 6, isintercepted by one of the faces of a rotating mirror 7 which is drivenby a motor 8. The function of the rotating mirror 7 is to impart ahorizontal scan di mension to the laser beam which is already scanningin the vertical direction. Inasmuch as the horizontal scan rate must bemuch slower, in the embodiment shown, than the vertical scan rate, therotating mirror 7 is provided with substantially more faces than therotating mirror 3, and the motor 8 turns at a much slower angular ratethan the corresponding motor 4. The light beam from the first stationarymirror 5 is directed onto the faces of the rotating mirror 7 slightly toone side of the axis of rotation such that the light reflected from thesuccessively passing faces of the rotating mirror 7 is redirected at anacute angle and continues to diverge until it impinges upon the surfaceof a second stationary concave mirror 9.

The second stationary mirror 9 is ideally ground as an off-axisparabola; however, it has been found that a spherical section, which ismuch easier to fabricate, is entirely adequate for most applications.The function of the second stationary mirror 9 is to redirect the laserbeam toward the planar target 10 and to bring the beam into focus at thesurface of the target. It will be observed that the second stationarymirror 9 accepts the incoming beam as if the exemplary focal point 6were an extension through the second rotating mirror 7 to a positionsubstantially in the plane defined by the planar target 10. Inasmuch asthe resulting raster sweep at the planar target 10 is in focusthroughout the nominal target area, those skilled in the art willunderstand that the laser beam is substantially normal to the planedefined by the target 10 and the second stationary mirror 9 such thatthe mirror 9 must be sufficiently large to accommodate the entirenominal target area.

Assuming that the rotating mirror 3 has moved in the direction indicateda few degrees, the beam will take the path to the target 10 indicated bythe solid lines which, it will be observed, strikes the first stationarymirror 5 at a lower point, focuses at a point 6', and impinges upon aface of the rotating mirror 7 from which it is reflected to the secondstationary mirror 9 at a position higher than the previously describedcondition.

Therefore. the beam is now focused by the second stationary mirror 9 onthe target 10 at a point directly above the point of impingementpreviously described. It is essential to note that the light bundleimpinges upon the reflective faces of the rotating mirror 7 at astationary position and that the light bundles passing from thestationary mirror 9 to the target 10 move parallel to achieve a linescan on the target 10, the lines being shifted horizontally inaccordance with the rate of rotation of the mirror 7.

it will be seen from a perusal of the single FIGURE and the foregoingdiscussion that planar scanning has been achieved utilizing relativelysimple and inexpensive components. Planar scanning of almost any practical size may be achieved by re-orienting the various components andutilizing a second stationary mirror 9 of adequate size. The field anglecan be adjusted by altering the positions of the two rotating mirrors.It will be observed that for the scan rates contemplated, a relativelysmall number of faces is required on the highspeed rotating mirror 3,and it is therefore possible to secure sharp definition between adjacentmirror faces with correspondingly low dead time because the laser beamimpinges on a very small area on both rotating mirrors thereby realizinga high off-to-on ratio. Another advantage of focusing the beam withinthe family of points exemplified by the point 6 is that the componentsare relatively small and therefore high frequency response can beachieved without undue difficulty caused by rotating bodies having highinertia.

Certain detail changes can be introduced to accom' modate the scansystem to certain operating requirements and environments. For example,the focal point 6 of the collimated beam reflected from the firststationary concave mirror 5 can be shifted by introducing a lens 12 intothe collimated light beam 1 issued from the laser 2 before it impingesupon the first rotating mirror 3. The lens 12, by focusing the beam 1 insuch a manner that the bundle is diverging when it impinges upon thestationary mirror 5 and is larger than the collimated beam issued fromthe source, 2 alters the focal length of the system in such a mannerthat the target 10 may be moved further from the mirror 9. Similarly, ifthe lens 12 is placed to provide a smaller light bundle impinging uponthe mirror 5, the target 10 may be moved closer to the stationary mirror9. In another contemplated configuration, the first rotating mirror 3may assume the form of the rotating mirror 7 rather than the pyramidalform disclosed in the single FIG- URE. The collimated beam from thelaser 2 would then preferably be disposed in the plane described by thevertically sweeping beam between the first rotating mirror 3 and thefirst stationary mirror 5. However, those skilled in the art willappreciate that the use of the pyramidal mirror 3 has the advantage ofbringing about a vertical sweep through an angle equal to the anglebetween adjacent faces whereas, multiface mirrors, such as the mirror 7,bring about a doubling of the swept angle.

it will be readily apparent that, if only linear scan is desired, thatthe laser beam may be directed at the mirror 7 from a position occupiedin the drawing by the mirror 5 or in such other position and withutilization of a converging lens, similar to the lens 12, to providefocus at the point 6. in this configuration, the rotating mirror 7redirects the beam to the stationary mirror 9 to provide repetitivehorizontal sweep along a line on the target 10.

While the principles of the invention have now been made clear in anillustrative embodiment, there will be immediately obvious to thoseskilled in the art many modifications of structure, arrangement,proportions, the elements, materials, and components, used in thepractice of the invention which are particularly adapted for specificenvironments and operating requirements without departing from thoseprinciples.

I claim:

1. Apparatus for providing uniform scan from a source of collimatedlight of any wave length comprising:

A. means for generating a collimated beam of light;

B. focusing means for focusing said beam such that it diverges afterpassing through a focal point;

C. a rotating mirror provided with a plurality of faces I equallycircumferentially disposed about its axis of rotation, said rotatingmirror being disposed to receive said diverging beam such that saiddiverging beam is redirected to sweep unidirectionally through apredetermined arc of a magnitude according to the number of faces onsaid rotating mirror;

D. a stationary mirror disposed to intercept and redirect said divergingbeam of light from said rotating mirror as a converging beam, saidstationary mirror having at least one finite focusing point; and

E. a planar target disposed generally at the line in which saidcollimated beam is substantially in focus.

2. Apparatus for providing a uniform raster scan on a planar target froma light source of any wavelength comprising:

A. means for generating a collimated beam of light;

B. a first rotating mirror having a plurality of equal faces, said firstrotating mirror being disposed in the path of said collimated beam suchthat said faces serially intercept and redirect said beam causing saidbeam to sweep repeatedly and unidirectionally through a firstpredetermined arc, the angle of said first predetermined are being of amagnitude according to the number of faces on said first rotatingmirror;

C. a first stationary mirror, said first stationary mirror beingdisposed in the path of said collimated beam of light as it sweepsthrough said first predetermined arc and being of sufficient dimensionto intercept and redirect said collimated beam of light as a convergingbeam as it sweeps through the entirety of said first predetermined are,said first stationary mirror having at least one predetermined focalpoint, said converging beam translating into a diverging beam afterpassing through said focal point;

D. a second rotating mirror provided with a plurality of faces equallycircumferentially disposed about its axis of rotation, said secondrotating mirror being disposed to receive said diverging beam of lightreflected from said first stationary mirror such that said divergingbeam is redirected to sweep unidirectionally through a secondpredetermined arc of a magnitude according to the number of faces onsaid second rotating mirror;

E. a second stationary mirror disposed to intercept and redirect saiddiverging beam of light from said second rotating mirror as a convergingbeam, said second stationary mirror having at least one finite focuspoint; and

F. a planar target disposed generally at the plane in which saidcollimated beam is substantially in focus.

3. The apparatus of claim 2 in which said first rotating mirror ispositioned beyond the focal point of said first stationary member suchthat said diverging beam of light impinging upon the faces of saidsecond rotating mirror remains stationary.

4. The apparatus of claim 3 in which the distance between the focalpoint of said first stationary mirror and the distance between thereflecting face of said second rotating mirror instantaneouslyintercepting said diverging beam to said second stationary mirror equalsthe focal length of said second stationary mirror.

5. The apparatus of claim 4 in which a converging lens is introducedbetween the beam issuing from said means for generating a collimatedbeam of light and said first rotating mirror thereby altering the systemfocal length.

1. Apparatus for providing uniform scan from a source of collimatedlight of any wave length comprising: A. means for generating acollimated beam of light; B. focusing means for focusing said beam suchthat it diverges after passing through a focal point; C. a rotatingmirror provided with a plurality of faces equally circumferentiallydisposed about its axis of rotation, said rotating mirror being disposedto receive said diverging beam such that said diverging beam isredirected to sweep unidirectionally through a predetermined arc of amagnitude according to the number of faces on said rotating mirror; D. astationary mirror disposed to intercept and redirect said diverging beamof light from said rotating mirror as a converging beam, said stationarymirror having at least one finite focusing point; and E. a planar targetdisposed generally at the line in which said collimated beam issubstantially in focus.
 2. Apparatus for providing a uniform raster scanon a planar target from a light source of any wavelength comprising: A.means for generating a collimated beam of light; B. a first rotatingmirror having a plurality of equal faces, said first rotating mirrorbeing disposed in the path of said collimated beam such that said facesserially intercept and redirect said beam causing said beam to sweeprepeatedly and unidirectionally through a first predetermined arc, theangle of said first predetermined arc being of a magnitude according tothe number of faces on said first rotating mirror; C. a first stationarymirror, said first stationary mirror being disposed in the path of saidcollimated beam of light as it sweeps through said first predeterminedarc and being of sufficient dimension to intercept and redirect saidcollimated beam of light as a converging beam as it sweeps through theentirety of said first predetermined arc, said first stationary mirrorhaving at least one predetermined focal point, said converging beamtranslating into a diverging beam after passing through said focalpoint; D. a second rotating mirror provided with a plurality of facesequally circumferentially disposed about its axis of rotation, saidsecond rotating mirror being disposed to receive said diverging beam oflight reflected from said first stationary mirror such that saiddiverging beam is redirected to sweep unidirectionally through a secondpredetermined arc of a magnitude according to the number of faces onsaid second rotating mirror; E. a second stationary mirror disposed tointercept and redirect said diverging beam of light from said secondrotating mirror as a converging beam, said second stationary mirrorhaving at least one finite focus point; and F. a planar target disposedgenerally at the plane in which said collimated beam is substantially infocus.
 3. The apparatus of claim 2 in which said first rotating mirroris positioned beyond the focal point of said first stationary membersuch that said diverging beam of light impinging upon the faces of saidsecond rotating mirror remains stationary.
 4. The apparatus of claim 3in which the distance between the focal point of said first stationarymirror and the distance between the reflecting face of said secondrotating mirror instantaneously intercepting said diverging beam to saidsecond stationary mirror equals the focal length of said secondstationary mirror.
 5. The apparatus of claim 4 in which a converginglens is introduced between the beam issuing from said means forgenerating a collimated beam of light and said first rotating mirrorthereby altering the system focal length.